During combustion in compression ignition internal combustion engines, above temperatures of approximately 2500° F. (1372° C.), nitrogen in air reacts with oxygen to produce oxides of nitrogen (NOx). Regulations and the like such as the so-called US07, US10, and Euro 5 regulations on emissions levels for heavy-duty, on-highway vehicles would substantially limit the amount of NOx that an engine can produce.
Exhaust gas recirculation (EGR) systems, which introduce a portion of the exhaust gases into the engine intake, dilute the oxygen in the incoming air charge and can lower combustion temperatures. Accordingly, EGR systems can be useful in reducing engine NOx emissions. The amount of EGR flow can be controlled by an EGR controller such as an EGR valve, a turbocharger, such as a variable geometry turbocharger (VGT), an exhaust backpressure device, an intake throttle, and the like, and, typically, by some combination of two or more of these controllers.
One problem with EGR systems is that they tend to drive up heat rejection. In order to introduce the EGR flow to the engine intake, it ordinarily must be cooled in an EGR cooler. The need for an EGR cooler typically means that the power unit, such as the engine of a vehicle, will require more radiator surface area, i.e., bigger radiators, and/or larger cooling fan. This, in turn, typically imposes limitations on vehicle designs, such as necessitating large front faces having poor aerodynamic characteristics.
Another problem with EGR systems is that they tend to increase fuel consumption. In addition to fuel consumption increases due to limits on vehicle aerodynamics and cooling fan power consumption, it is necessary to create sufficient exhaust back pressure to force the exhaust gas through the cooler and into the intake system. The mixture of exhaust gas and fresh air must also be pumped into the combustion chamber. These “pumping” losses can be substantial.
Yet another problem with EGR systems is that they tend to lower power density, i.e., power output is lower relative to displacement.
Other techniques for limiting NOx emissions include use of exhaust aftertreatment (EAT) systems, such as reduction agent introduction systems for introducing reduction agents such as urea or ammonia, as well as hydrocarbons or hydrogen into the exhaust stream, together with selective catalytic reduction (SCR) catalysts (urea, hydrocarbon (HC), ammonia, hydrogen, alcohols, etc.) or other technology such as lean NOx adsorbers (LNA). As explained in U.S. Pat. No. 6,871,490, which is incorporated by reference, as the exhaust stream and reduction agent passes through the SCR catalyst, the reduction agent reacts with the NOx to reduce the NOx to molecular nitrogen and water, thereby reducing the NOx emissions from the engine.
It is desirable to maintain NOx emissions at or below predetermined levels, such as those set in the various regulations. It is also desirable to minimize EGR use and thereby reduce heat rejection, improve fuel consumption, and increase power density.
In accordance with an aspect of the present invention, an engine with an emissions control arrangement comprises an engine comprising an intake and an exhaust system, an EGR system comprising a conduit between the exhaust system and the intake and an EGR controller between the exhaust system and the conduit, the EGR controller being adapted to control EGR flow from the exhaust system to the intake, a reduction agent introduction system adapted to introduce a reduction agent into the exhaust system, and a controller arranged to adjust EGR flow and reduction agent introduction as functions of each other.
In accordance with another aspect of the present invention, a method of controlling engine emissions comprises adjusting EGR flow from an exhaust system to an intake of the engine and adjusting introduction of a reduction agent into the exhaust system, the EGR flow and reduction agent introduction being adjusted as functions of each other.